Polymerization process using surface heat treated silica-containing catalyst base

ABSTRACT

A silica base for a chromium olefin polymerization catalyst is subjected to a high temperature treatment as for instance by passing the silica through a flame in order to alter the surface characteristics. The chromium can be present during the flame treatment or added later. The result is a catalyst capable of giving a broader molecular weight distribution polymer which is of particular value with the silica-titania coprecipitated catalyst used in slurry olefin polymerization.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of copending application Ser. No. 303,302, filedSept. 17, 1981, U.S. Pat. No. 4,378,306.

BACKGROUND OF THE INVENTION

This invention relates to a treatment of silica catalyst bases.

Chromium oxide catalysts on a silica-containing support have long beenused to prepare olefin polymers in a hydrocarbon solution to give aproduct having excellent characteristics from many standpoints. Ascommercialization of this process became widespread, it became evidentthat polymer could be produced more economically utilizing a slurrysystem achieved by carrying out the polymerization at a temperature lowenough that the resulting polymer did not go into solution in thediluent used. This process, frequently referred to as a particle formprocess, while being less complex has certain drawbacks. For one thing,certain control operations which are easily carried out in the solutionprocess are considerably more difficult in the particle form process.Particularly with regard to the molecular weight, it is a simple matterin the solution process to control molecular weight by changing thetemperature, with lower molecular weight (higher melt flow) polymerbeing obtained at the higher temperatures. However, in the slurryprocess, this technique is inherently limited since any effort toincrease the melt flow to any appreciable extent by increasingtemperature would cause the polymer to go into solution and thus destroythe slurry or particle form process. This problem has been largelycircumvented by catalyst modifications which inherently give a highermelt flow polymer under a given set of conditions. For instance, a smallamount of titanium can be coprecipitated with the silica to produce acogel base and admixed with a chromium compound to provide a catalystwhich has the inherent capability of giving higher melt flow polymer.This, however, can create an additional problem in that polymer having arelatively narrow molecular weight distribution is generally produced.Such polymers are ideally suited for some applications but for otherapplications, a broader distribution is essential. A broaderdistribution could be obtained by simply forming two different types ofcatalysts and physically mixing them but this would create two separatepolymers which can segregate and thus create a lack of homogeneity inthe final product.

SUMMARY OF THE INVENTION

It is an object of this invention to produce a catalyst capable ofgiving broad molecular weight distribution;

it is a further object of this invention to take advantage of theinherent high melt flow capability of silica-titania-cogelled catalystwithout a sacrifice in molecular weight distribution of polymersproduced therewith;

it is a further object of this invention to provide an improved olefinpolymerization catalyst; and

it is still yet a further object of this invention to provide animproved olefin polymerization process.

In accordance with this invention particulate silica-containing materialis subjected to a brief but intense heat treatment so as to alter thesurface of the particles.

BRIEF DESCRIPTION OF THE DRAWING

The drawing, forming a part hereof, shows the consistent improvement inHLMI/MI ratio for catalyst made in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The silica-containing material treated in accordance with this inventionis generally composed of 80 to 100 weight percent silica (either with orwithout chromium as will be discussed hereinafter), the remainder, ifany, being selected from alumina, boria, magnesia, thoria, titania,zirconia, or mixtures thereof. The silica-containing material canconsist essentially of silica and no more than 0.2 weight percentalumina but is preferably a silica-titania cogel formed by precipitatingthe silica in the presence of a sufficient amount of a titanium compoundto give 0.1 to 20, preferably 1 to 12 weight percent titanium in thefinal dried or calcined catalyst based on the weight of the support. Atergel formed by coprecipitating silica, titania and a chromium compoundcan also be treated in accordance with this invention. Large poresilicas particularly suitable for treatment in accordance with thisinvention are described in Dietz, U.S. Pat. No. 3,887,494 (June 3, 1975)which covers the preparation of silica/titania cogels and Witt, U.S.Pat. No. 3,900,457 (Aug. 19, 1975) covering the preparation of syntheticsilica, the disclosures of which are hereby incorporated by reference.

Inherently, tergels formed by coprecipitating the silica, titanium, andchromium have the chromium present during the surface heat treatment. Inthe case of silicas which do not have chromium coprecipitated therewith,the chromium can either be incorporated prior to the heat treatmentpreferably by means of an aqueous solution of a chromium compound beingimpregnated into the hydrogel stage or a non-aqueous solution of achromium compound being impregnated into the xerogel stage as is wellknown in the art.

Alternatively, the chromium can be added after the heat treatment of thesurface. In this latter case, if the starting material is a large poresilica particularly a silica-titania cogel, the chromium incorporationis preferably carried out by means of an anhydrous solution or if anaqueous solution is used, the water is removed by azeotropicdistillation or washing with a volatile water miscible organic compoundso as to minimize damage to the interior pores of the silica. Such waterremoval techniques are broadly shown in the Dietz and Witt patentsdescribed hereinabove.

However the chromium is incorporated, the final catalyst contains aconventional amount of chromium which is generally an amount within therange of 0.001 to 10, preferably 0.1 to 5, more preferably about 1weight percent based on the weight of the dried silica-containingmaterial (xerogel).

The heat treatment in accordance with this invention is preferablycarried out simply by passing the particulate silica-containing materialthrough a flame, e.g. oxy-hydrogen, natural gas, or other conventionalflame sources. This can be done either by the use of a gun effectwherein the particulate silica-containing material is passed axiallythrough a burner into the flame or the silica-containing material cansimply be dropped through the flame. Any heat treatment which willachieve the desired temperature can be utilized including the use of afurnace or a blast of hot gas, the essential feature being that thetreatment zone has a very high temperature and the residence time isvery short so as to allow sintering or melting of the outer surface soas to reduce the surface area and pore volume, generally by at least 50percent, while leaving the interior of the particles essentiallyunchanged insofar as pore volume and surface area are concerned. Toachieve this, temperatures of 1200° to 4000° C., preferably about 1800to 2500°C. are utilized in conjunction with residence times within therange of about 0.1 to 3, preferably 0.5 to 2 seconds. It is readilyapparent that the higher temperatures and shorter residence time wouldgenerally go together and vice versa.

After the surface treatment, the chromium-containing catalyst isactivated by heating in an oxygen-containing ambient such as airgenerally at a temperature of 500° to 1000° C., preferably 600° to 800°C. and a time of at least 5 minutes, preferably 0.5 to 24 hours, morepreferably 1 to 4 hours in a conventional manner for activating silicasupported chromium catalysts. The catalysts can also be given areduction and reoxidation treatment. These treatments shown in McDanielet al., U.S. Pat. No. 4,182,815 (Jan. 8, 1980), the disclosure of whichis hereby incorporated by reference.

The catalysts produced in accordance with this invention can be used topolymerize at least one mono-1-olefin containing 2 to 8 carbon atoms permolecule, such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene,and 1-octene. Such catalysts are of particular applicability inproducing ethylene homopolymers and copolymers of ethylene and one ormore comonomers selected from 1-olefins containing 3 to 8 carbon atomsper molecule, such as propylene, 1-butene, 1-pentene, 1-hexene, and1-octene. These polymers can be produced by solution polymerization,slurry polymerization, and gas phase polymerization using conventionalequipment and contacting processes. Contacting of the monomer ormonomers with the catalyst can be effected by any manner known in theart of solid catalysts. One convenient method is to suspend the catalystin an organic medium and to agitate the mixture to maintain the catalystin suspension throughout the polymerization process. Typically,predominantly ethylene copolymers are prepared using 0.5 to 20 weightpercent comonomer as described above in the feed, preferably sufficientcomonomer being used to give 97 to 99.6 weight percent polymerizedethylene units in the resulting copolymer.

EXAMPLE I--CATALYST PREPARATION

A commercially obtained catalyst comprising chromium oxide supported oncoprecipitated silica-titania (cogel catalyst) was employed in the formreceived. The catalyst consisted of chromium oxide in an amount givingabout 1 weight percent chromium, titanium dioxide in an amount givingabout 2.5 weight percent titanium, and the balance as silica. Ahorizontally disposed, laboratory-size glass blowing torch having anozzle diameter of about 1/8 inch (0.32 cm) and a flame length of about18 inches (46 cm) was used as the flame source. A metal foil was placedbelow the flame to collect catalyst passing through it. The collectingfoil was arbitrarily divided into three zones as measured from the endof the burner nozzle. Zone A ranged from 0 to 3 inches (0 to 7.6 cm),zone B ranged from 3 to 9 inches (7.6 to 23 cm) and zone C was greaterthan 9 inches. The average pass time for catalysyt traversing the flameranged from less than 1/10 second (zone A) to greater than 3 seconds(zone C) with about 0.5 to about 1 second for zone B.

In each run, portions of the catalyst was sprinkled through the flame atthe end of burner nozzle. Catalyst falling through the flame wascollected in zones A, B and C. Catalyst in zone A received too littleflame treatment and catalyst in zone C received the most treatment (toomuch) since it was carried in the flame longer and it was exposed to thehigh temperatures a longer period of time. The flame temperature wasestimated to be about 2000° C. at its hottest point ranging down toabout 1200° C. at the end.

The collected, treated catalyst was subsequently activated forpolymerization by calcination in a fluidized bed with dry air for 5hours at 1600° F. The activated catalyst was recovered and stored in dryair until ready for use.

The untreated control catalyst was activated for 5 hours at 1400° F.(760° C.), recovered and stored until ready for use as above. A loweractivation temperature was required to provide the control catalyst withsimilar melt index capability to that of treated catalyst at a givenreactor temperature. It is very important to hold this variable constantwith supported chromium oxide catalysts since experience has shown thatwith ethylene polymers made with such catalysts, the ratio of high loadmelt index to melt index (HLMI/MI) is sensitive to reactor temperature.The ratio is thought to relate to polymer molecular weight distribution,the greater the ratio the broader the molecular weight distribution.HLMI is determined in accordance with ASTM D 1238-65T (Condition F) andMI is determined in accordance with ASTM D 1238-65T (Condition E), witheach flow value expressed as grams per 10 minutes.

A weighed sample of each catalyst in the 0.03 to 0.06 g range wasemployed in the particle form polymerization of ethylene in a stirred,stainless steel reactor of one gallon capacity (3.8 L) containing 1.2 Lof isobutane diluent. A reactor pressure of 565 psia (4.52 MPa) was usedat the temperature specified. Ethylene was supplied on demand to thereactor during the reaction from a pressurized reservoir. Each run wasconducted for sufficient time to provide an estimated 5,000 gramspolymer per gram catalyst. The recovered product was dried, stabilizedwith a conventional antioxidant such as 2,6-di-t-butyl-4-methylphenol,and the MI and HLMI values determined.

The frame treatment used in catalyst preparation and the polymerizationresults obtained are given in the table.

                                      TABLE                                       __________________________________________________________________________     Ethylene Polymerization                                                               Catalyst        Run Polymer                                          Run No.                                                                          Reactor Temp. °C.                                                            Collection Zone                                                                     Activated Color                                                                         Time, min.                                                                        Weight, g                                                                           MI                                                                               ##STR1##                                                                           Remarks                            __________________________________________________________________________    1  102   C     green, orange tinge                                                                     --  about 5                                                                            -- --   Inactive                            2  110   C     green, orange tinge                                                                     --  about 2                                                                            -- --   Inactive                            3  102   B     orange, green tinge                                                                     45  185.5                                                                               0.44                                                                            113  Invention                           4  106   B     orange, green tinge                                                                     43  155.3                                                                              1.5                                                                              63   Invention                           5  110   B     orange, green tinge                                                                     70  238  2.6                                                                              64   Invention                           6  102   --    orange, green tinge                                                                     42  135  0.4                                                                              71   Control                             7  106   --    orange, green tinge                                                                     45  172  1.6                                                                              47   Control                             8  110   --    orange, green tinge                                                                     43  170  2.4                                                                              42   Control                             __________________________________________________________________________

Catalyst from collection zone A was little influenced by its briefexposure to the flame. Its color after activation was identical to thecolor of the control catalyst. Polymer was not made with that catalyst.Catalyst from collection zone C was overtreated. Its color afteractivation was green and it had poor catalytic properties as the resultsin runs 1, 2 demonstrate. However, invention catalyst from collectionzone B, utilized in runs 3, 4, and 5 had received enough flame treatmentto at least partially sinter the exterior of the particles whileessentially leaving the interior of the particles unchanged. Thetreatment thus afforded a single catalyst capable of producing polymerhaving average molecular weights differing in magnitude depending uponwhether the catalytically active sites are located in the interior orexterior portions of the catalyst. The polymer produced with theinvention catalyst exhibits a much broader molecular weight distributionthan that produced with untreated catalyst when the comparison is madewith polymer made at the same reactor temperature and having about thesame melt index.

While this invention has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

We claim:
 1. A polymerization process comprisingcontacting at least onemono-1-olefin having 2 to 8 carbon atoms per molecule in a reaction zoneunder polymerization conditions with a catalyst produced by a processcomprising: subjecting a particulate silica-containing material to aheat treatment for a time and at a temperature sufficient to sinter theoutside of the particles of said silica-containing material withoutsignificantly affecting the inside of the particles, saidsilica-containing-material either having a chromium compound containedtherein during said heat treatment or having a chromium compound addedthereto, and activating by a process comprising heating saidsilica-containing material also containing said chromium compound in anoxygen-containing ambient; and recovering a polymer.
 2. A methodaccording to claim 1 wherein said at least one mono-1-olefin is selectedfrom ethylene, propylene, 1-butene, 1-pentene, 1-hexene, and 1-octene.3. A method according to claim 1 wherein said at least one mono-1-olefincomprises ethylene.
 4. A method according to claim 1 wherein saidpolymer is an ethylene homopolymer or a copolymer having 97 to 99.6weight percent polymerized ethylene units.
 5. A method according toclaim 1 wherein said polymerization is carried out under slurryconditions.
 6. A method according to claim 1 wherein said thus heattreated silica-containing material containing said chromium compound isactivated by heating in air at a temperature of 600°-800° C. for a timewithin the range 0.5 to 24 hours.
 7. A method according to claim 6wherein said heat treatment is effected by passing saidsilica-containing material already containing said chromium compoundthrough a flame, and wherein said silica-containing material is asilica-titania cogel.
 8. A polymerization process comprising:containingat least one mono-1-olefin having 2 to 8 carbon atoms per molecule in areaction zone under polymerization conditions with a catalyst producedby a process comprising: passing a particulate silica-containingmaterial through a zone at a temperature within the range of 1200° to4000° C. for a residence time within the range of 0l.1 to 3 seconds,said silica-containing material either containing a chromium compoundduring said passing or having a chromium compound thereafter addedthereto, and activating by a process comprising heating saidsilica-containing material also containing said chromium compound in anoxygen-containing ambient; and recovering a polymer.
 9. A methodaccording to claim 8 wherein said at least one mono-1-olefin is selectedfrom ethylene, propylene, 1-butene, 1-pentene, 1-hexene, and 1-octene.10. A method according to claim 8 wherein said at least onemono-1-olefin comprises ethylene.
 11. A method according to claim 8wherein said polymer is an ethylene homopolymer or a copolymer having 97to 99.6 weight percent polymerized ethylene units.
 12. A methodaccording to claim 8 wherein said polymerization is carried out underslurry conditions.
 13. A method according to claim 8 wherein the thusheat treated silica-containing material containing said chromiumcompound is activated by heating in air at a temperature within therange of 600° to 800° C. for a time within the range of 0.5 to 24 hours.14. A method according to claim 8 wherein said silica-containingmaterial is a silica-titania-chromium tergel.
 15. A method according toclaim 8 wherein said silica-containing material is a silica-titaniacogel containing 1 to 12 weight percent titanium.
 16. A method accordingto claim 15 wherein said chromium compound is present during saidpassing.
 17. A method according to claim 16 wherein said chromiumcompound is introduced by combining an aqueous solution of a chromiumcompound selected from CrO₃, chromium acetate and chromium nitrate withsaid cogel at a hydrogel stage thereof and said cogel is formed byadding sodium silicate to sulfuric acid containing a titanium compound,aging, washing with water to remove sodium ions and removing water bycontact with one of ethyl acetate, or a 5 or 6 carbon atom alcohol. 18.A method according to claim 17 wherein said zone of 1200° to 4000° C. isa flame and said time range is 0.5 to 2 seconds.